1. Field of the Invention
The present invention relates to a reproducing circuit for use with a magnetoresistive (MR) head, and more particularly to a reproducing circuit for use with an MR head which requires no sensing current source for reducing electric power consumption.
2. Description of the Prior Art
MR heads as well as ordinary inductive heads are used as reproducing heads for use with hard disk drives (HDD). The MR heads have a thin magnetic film whose resistivity is varied by a magnetic field applied from a magnetic medium, and the variation in the resistivity is detected as a reproduced output voltage. The MR heads are also widely used as high-density recording and reproducing heads for use with digital audio tape recorders because the MR heads are of a high output capability, suffer a low crosstalk, and are not speed-dependent.
One known MR head, a shielded MR head, is shown in FIG. 1 of the accompanying drawings.
The shielded MR head shown in FIG. 1 comprises a pair of shield cores 11 defining a gap 12 therebetween which accommodates an MR element 13 integral with a signal conductor 14, and a bias conductor 15 parallel to the signal conductor 14. A signal magnetic field applied from a magnetic medium is directly picked up by the MR element 13.
The MR head shown in FIG. 1 is of the current bias type. Another known MR head is of the shunt bias type which does not require the bias conductor 15. In the MR head of the shunt bias type, a magnetic field is applied by an MR current itself which flows in the signal conductor 14.
The above MR head is illustrated as a reproducing head. If the MR head is used in a recording and reproducing head assembly, then the MR head and a thin-film inductive recording head may be superimposed or juxtaposed on the same substrate as a composite head arrangement, or the MR head may be integrally combined with an independent recording head.
An equivalent circuit of a reproducing circuit for use with the current-bias-type MR head is illustrated in FIG. 2 of the accompanying drawings. As shown in FIG. 2, an MR (sensing) current is supplied from a current source 2 to an MR element 13 of the current-bias-type MR head, denoted at 1, and a variation in the resistance of the MR element 13 which is caused by a signal magnetic field applied from a magnetic medium is picked up as a voltage. At the same time, in order to cause the MR element 13 to operate linearly, a bias current is supplied from a bias current source 16 to a bias conductor 15 to apply a bias magnetic field to the MR element 13.
The bias conductor 15 and the MR element 13 have grounded terminals. The voltage produced across the MR element 13 is applied through a DC blocking capacitor 3 as an unbalanced output voltage to a reproducing amplifier 4, which amplifies the applied signal.
The capacitance of the DC blocking capacitor 3 is selected to pass the supplied input signal to the extent that the error rate will not be lowered. For example, the capacitance of the DC blocking capacitor 3 is selected in the range of from about 0.01 .mu.F to 0.1 .mu.F depending on the bit rate.
A reproducing circuit for use with the shunt-bias type MR head is similar to the reproducing circuit shown in FIG. 2 except that the bias current source 16 and the bias conductor 15 are dispensed with.
FIG. 3 of the accompanying drawings shows a conventional recording and reproducing IC (W/R circuit) for use in a recording and reproducing apparatus which incorporates an integral assembly of an MR reproducing head and an inductive recording head. In FIG. 3, an MR head 1 is of the same circuit arrangement as the equivalent circuit shown in FIG. 2, and is combined with a recording head 17 and mounted on the tip end of a gimbal or the like.
The recording and reproducing apparatus, denoted at 18, comprises a reproduced signal processor 19 in the form of an IC and the MR head 1. The reproduced signal processor 19 has a constant voltage source 22 and a W/R circuit 33. The W/R circuit 33 has a reproducing amplifier 4 and a recording amplifier 21. The reproducing amplifier 4 has an output terminal connected to a reproduced output terminal T13, and the recording amplifier 21 has an input terminal connected to a recording input terminal T14. The reproduced output terminal T13 outputs a reproduced signal that has been picked up by the MR head 1, and the recording input terminal T14 is supplied with a recording signal to be recorded on a recording medium. The recording amplifier 21 has balanced output terminals connected to recording output terminals T11, T12 which are connected respectively to recording head terminals T5, T6 coupled to a recording coil of the recording head 17 combined with the MR head 1.
The terminals of the MR element 13 are connected to respective MR element terminals T2, T3, and the terminals of the bias conductor 15 are connected to respective bias conductor terminals T1, T4. The MR element terminal T3 is connected to a ground input terminal T9 of the reproduced signal processor 19, and the bias conductor terminal T4 is connected to a ground input terminal T10 of the reproduced signal processor 19.
The bias conductor terminal T1 is connected to a bias output terminal T7 that is connected to the constant voltage source 22 through a variable resistor 23 for adjusting a bias current.
The MR element terminal T2 is connected to a reproducing amplifier input terminal T8 which is connected to the input terminal of the reproducing amplifier 4 in the W/R circuit 33 through a DC blocking capacitor 3. A variable resistor 24 for adjusting an MR current is connected between the output terminal of the constant voltage source 22 and the reproducing amplifier input terminal T8.
In the conventional recording and reproducing apparatus 18 combined with the MR head 1, it is necessary to optimize a bias current supplied to the bias conductor 15 in order to obtain a reproduced signal with reduced distortions from the MR element 13. Therefore, a current from the constant voltage source 22 is supplied to the variable resistor 23 which adjusts the bias current.
The MR current supplied to the MR element 13 has to be set to an optimum value because it differs depending on the type and lot of the MR head 1. If the MR head 1 is of the shunt bias type, then it is necessary to adjust the MR current supplied to the MR element 13 in order to produce a reproduced waveform with reduced distortion. Therefore, the MR current is adjusted by the variable resistor 24.
As described above, the reproducing circuit shown in FIG. 2 requires the current source 2 for supplying the sensing current to the MR element 13 and also the bias current source 16 for supplying the bias current to the bias conductor 15. The recording and reproducing circuit shown in FIG. 3 requires the constant voltage source 22 and the variable resistor 24 for adjusting the MR current, resulting in increased current consumption in the reproduced signal processor 19.
If the MR head 1 shown in FIG. 2 is of a multi-head structure, then it is necessary to employ individual variable resistors 23, 24 for adjusting the MR and bias currents that are supplied to MR elements and bias conductors for respective channels. Accordingly, the number of variable resistors used is increased, and so is the number of locations where their adjustments have to be made. Another problem is that the power consumption is increased because the head of each channel is supplied with a current at all times.
In the case where the W/R circuit 33 is used with an HDD, since the capacitance of the DC blocking capacitor 3 is of such a value that it cannot be contained in the IC of the W/R circuit 33, the reproduced signal processor 19 is required to have a terminal for connection to an external DC blocking capacitor 3 and a terminal for being supplied with a signal to supply a sensing current. Consequently, if the W/R circuit 33 is of a multi-channel circuit arrangement, then the number of terminals added is increased, and also the number of external DC blocking capacitors is increased.